Effective therapy of human lymphoma xenografts with a novel recombinant ribonuclease/anti-CD74 humanized IgG4 antibody immunotoxin

Abstract

Ranpirnase (Rap) is a cytotoxic ribonuclease (RNase) isolated from frog oocytes. Here we describe high antitumor activity of a novel immunotoxin, 2L-Rap-hLL1-γ4P, composed of 2 Rap molecules, each fused to the N terminus of the light chain of hLL1, an internalizing anti-CD74 humanized antibody. To reduce unwanted side effects, the constant region of hLL1 was changed from γ1 to γ4 and further to γ4P by replacing serine228 to proline to prevent the formation of a half immunoglobulin G (IgG) common for IgG4. In vitro, 2L-Rap-hLL1-γ4P retained RNase activity, specific binding to CD74, and was significantly more potent against CD74+ cell lines (Daudi, Raji, and MC/CAR) than naked hLL1. In vivo, the pharmacokinetic profile of 2L-Rap-hLL1-γ4P was similar to that of naked hLL1. The maximum tolerated dose of 2L-Rap-hLL1-γ4P in severe combined immunodeficient mice (SCID) or BALB/c mice was 50 μg per mouse. In Raji and Daudi Burkitt lymphoma xenograft models, treatment with a single 5 to 50 μg dose of 2L-Rap-hLL1-γ4P, given as early or delayed treatment, resulted in cures of most animals. Treatment with 2L-Rap-hLL1-γ4P was significantly better than all controls, including saline, naked hLL1, and nonspecific immunotoxin. In conclusion, 2L-Rap-hLL1-γ4P demonstrated excellent in vitro and in vivo efficacy and thus merits further consideration as a therapeutic for CD74+ tumors.